The treatment of hides and skins to form leather involves a number of interdependent chemical and mechanical operations. These operations may be divided into a sequence of "wet end" steps followed by a sequence of"dry" steps. A description of each of these operations is provided in Fundamentals of Leather Manufacturing, Prof Dr. Heidemann (Eduard Roether KG, 1993). The primary tanning operation involves the treatment of the hide to preserve it and form useful leather. Chrome tanning salts are well known and widely used for this purpose. Chrome-tanned hides or skins are known in the art as `wet blue leather`. In order to produce a uniform piece of leather with the required physical and aesthetic properties, a second tanning step, known as `retanning`, is employed. Retanning can be accomplished using a variety of naturally derived materials including extracts from vegetables or plants, and synthetic tanning agents known as "syntans", or combinations thereof After or during retanning, the leather can be colored and fatliquored. The present invention relates to the wet end operations which take place after primary tanning, namely retanning and fatliquoring.
A number of publications have proposed various copolymers for treating leather during tanning and retanning, addressing the problem of making treated leather more water resistant or completely waterproof
EP-A-372 746 discloses a method and process for treating leather utilizing selected amphiphilic copolymers for improving the strength, temper and water resistance of the leather. The amphiphilic copolymers are formed from a predominant amount of at least one hydrophobic monomer and a minor amount of at least one copolymerizable hydrophilic monomer. The application states that the process may be particularly useful as a one step substitute for conventional retanning and fatliquoring treatment steps.
EP-A-682 044 discloses copolymers comprising ethylenically unsaturated dicarboxylic acid anhydrides, long chain olefins and fluorolefins. Leathers treated with these polymers are shown to yield good waterproofness results according to the Bally-Penetrometer test.
U.S. Pat. No. 5,124,181 discloses copolymers which contain (a) from 50 to 90% by weight of C.sub.8 to C.sub.40 alkyl methacrylates, vinyl esters of C.sub.8 to C.sub.40 carboxylic acids or mixtures thereof and (b) from 10 to 50% by weight of monoethylenically unsaturated C.sub.3 to C.sub.12 carboxylic acids, monoethylenically unsaturated dicarboxylic anhydrides, monoesters or monoamides of monoethylenically unsaturated C.sub.4 to C12 dicarboxylic acids, amides of C.sub.3 to C.sub.12 monocarboxylic acids or mixtures thereof as copolymerized units and which have molecular weights of from 500 to 30,000. The copolymers are used in at least partially neutralized form in aqueous solution or dispersion for making leather and furs water repellent.
WO 94/01587 discloses water-dispersible and/or water-emulsifiable co-oligomers containing (a) fatty crotonates; (b) radically copolymerizable, hydrophilic, ethylenically unsaturated acids and/or their anhydrides; and possibly (c) minor amounts of other copolymerizable comonomers. These co-oligomers are used as amphiphilic agents for greasing leather and pelts.
Despite the various publications, employing different combinations of hydrophilic and hydrophobic monomers to obtain waterproofing properties of leather, there is still a need for further improvement particular under demanding conditions.